fix(firestore, android): clean up Android transaction listeners on completion - #18475

Description

Fixes an Android memory leak in cloud_firestore transactions where native transaction state was retained after runTransaction completed.
Previously, each Android transaction registered transaction-specific state including an EventChannel, StreamHandler, transaction handler, and transaction reference, but that state was only cleared during full plugin cleanup. Apps that ran many transactions over time could therefore retain completed transaction resources indefinitely.
This change adds per-transaction cleanup when the transaction stream completes, removing the associated transaction, event channel, stream handler, and transaction result handler.

Related Issues

Fixes #18474

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Breaking Change

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